1. Field of the Invention
The present invention relates to eye examining and testing instruments and, more particularly, to an apparatus for measuring the torsional rotation of an eyeball about the visual axis by means of an afterimage.
2. Description of the Prior Art
Medical and scientific studies have shown that measurements of torsional rotation of an eyeball about the visual axis are useful in identifying the source of certain sensori-neural problems and physical dysfunctions. However, measurements of ocular torsion are not currently utilized in medical applications due to the difficulty of observing ocular torsion and the inapplicability of techniques currently used to determine other types of eyeball motion. No apparatus has been devised which is sufficiently accurate, convenient, and economical for widespread routine clinical application.
Approaches to the measurement of ocular torsion are generally classified as either objective or behavioral. Objective methods utilize the natural or artificial landmarks on the surface of the eyeball in conjunction with an external referent to quantify changes in eyeball position. Reliable objective measurements have been obtained by the use of photographic records of natural landmarks; however, that technique is too time-consuming and costly for widespread routine clinical application. Behavioral methods generally measure ocular torsion by means of an afterimage: the phenomenon that when a person stares at an object for a period of time or sees a bright flash of light, an image of the object or flash of light can be seen afterwards. In afterimage testing, the retina is first imprinted with a line or a cross, a new stimulus condition is then set, and the patient is asked to report the orientation of the afterimage.
Typical of the prior art measurement techniques is the afterimage technique of Jongkees and Groen published in the Journal of Laringology and Otology, vol. 64, pages 135-140, 1950. A patient was instructed to focus on a line of vertical light for one minute to create an afterimage. Then, the patient's head was rotated about the dorsoventral axis by means of a special rotatable chair and the patient was asked to assist the tester to indicate the position of the afterimage on a separate white target screen. The tester held a ruler in front of the screen and rotated it according to the patient's instructions until the patient stated that the ruler was aligned with the afterimage. A similar technique was utilized by applicant Milburn in 1970 wherein reflected light from a flashbulb was substituted to imprint the afterimage.
Both the objective and behavioral techniques of the prior art have been too costly, inaccurate, or inconvenient for anything except very limited experimental use. It is therefore an object of the present invention to provide an apparatus for accurately and conveniently measuring ocular torsion and which is suitable for routine clinical application.